The present invention relates to pulse position modulation. More particularly, the present invention relates to detecting optical pulse position modulated signals using coherent correlation.
Many satellite and terrestrial optical communication systems require transmission of analog optical signals. The straightforward way to address this need is to modulate the amplitude of an optical carrier. This approach, however, suffers from poor signal-to-noise ratio (SNR). It is well known that broadband modulation techniques, which utilize higher bandwidth than that of the transmitted waveform, may improve the SNR over that achieved with amplitude modulation. Pulse Position Modulation (PPM) is one of these techniques. In PPM, a temporal shift in the pulse position represents a sample of the transmitted waveform. The improvement in SNR near the Nyquist sampling frequency of a pulse position modulated signal over an amplitude modulated signal is shown below:
SNRppmxe2x88x9dSNRam(tp/xcfx84)2
where tp is the temporal spacing between unmodulated pulses and xcfx84 is the pulse duration, respectively.
Conventional detection or demodulation of analog PPM optical signals, though, suffers from poor SNR at low frequencies. PPM signals are usually demodulated from the optical to electronic domain by a photodiode followed by a lowpass filter (LPF) that converts pulse position modulation to amplitude modulation. Such a demodulation technique is not capable of recovering the DC component, since the DC component is represented by a constant temporal shift of all pulses from their unmodulated positions. Moreover, the demodulated signals after the lowpass filter have very low amplitude at low frequencies. The amplitude increases linearly with frequency up to the Nyquist limit. Such frequency-dependent distortion is corrected by an integration circuit, which amplifies low-frequency noise accordingly, resulting in decreased SNR performance.
There exists a need in the art of a method and apparatus for detecting analog PPM optical signals at very low frequencies including DC. Moreover, the method and apparatus must provide SNR performance that does not degrade at lower frequencies.
An object of the present invention is to provide a method and apparatus for detecting a pulse position modulated optical signal at very low frequencies including DC. An additional object of the present invention is to provide pulse position modulation optical signal detection that does not exhibit degraded Signal-to-Noise performance at low frequencies.
A method and apparatus for detecting pulse position modulated optical signals is provided by an embodiment of the present invention having a clock source providing equally spaced optical clock pulses, a continuous wave optical source producing a continuous wave optical signal having a specific frequency, two coherent wavelength converters, and an optical correlator. An optical signal containing a pulse position modulated signal controls one coherent wavelength generator to produce a first stream of coherent pulses, preferably top hat shaped, at the frequency of the continuous wave. The clock source controls the other coherent wavelength generator to produce a second stream of coherent pulses, again preferably top hat shaped, also at the frequency of the continuous wave. The two pulse streams are cross-correlated by the optical correlator to produce an output whose value is proportional to the cross-correlation product.
An alternative embodiment of the present invention has a means for receiving an optical signal that contains both an optical pulse position modulated analog signal component and a clock signal component of equally spaced optical pulses, a means for splitting the optical signal into its two separate optical signal components, a continuous wave optical source producing a continuous wave optical signal having a specific frequency, two coherent wavelength converters, and an optical correlator. The optical signal containing the pulse position modulated signal controls one coherent wavelength generator to produce a first stream of coherent pulses, preferably top hat shaped, at the frequency of the continuous wave. The optical signal containing the clock signal of equally spaced optical pulses controls the other coherent wavelength generator to produce a second stream of coherent pulses, again preferably top hat shaped, also at the frequency of the continuous wave. The two pulse streams are cross-correlated by the optical correlator to produce an output whose value is proportional to the cross-correlation product.
Another embodiment of the present invention provides an apparatus for detecting temporal displacement between optical pulses in a first optical signal and optical pulses in a second optical signal wherein the first optical signal is synchronized with the second optical signal, and the apparatus comprises: a continuous wave optical source, a first coherent wavelength converter having inputs responsive to the first optical signal and to the continuous wave optical signal and producing a first coherent pulsed output; a second coherent wavelength converter having inputs coupled to the second optical signal and to the continuous wave optical signal and producing a second coherent pulsed output; and a coherent correlator having inputs coupled to the outputs of the first coherent wavelength converter and the second coherent wavelength converter and producing an output proportional to the first coherent pulsed output correlated with the second coherent pulse output. This output is also proportional to the temporal displacement between the optical pulses in the two streams.
Another embodiment of the present invention provides a method for detecting temporal displacement between optical pulses in a first optical signal and optical pulses in a second optical signal wherein the first optical signal is synchronized with the second optical signal, and the method comprises the steps of: generating a continuous wave optical signal; applying the continuous wave optical signal to a first coherent wavelength generator; providing the first optical signal to the first coherent wavelength generator; controlling the first coherent wavelength generator with the first optical signal so as to produce a first coherent pulse stream; applying the continuous wave optical signal to a second coherent wavelength generator; providing the second optical signal to the second coherent wavelength generator; controlling the second coherent wavelength generator with the second optical signal so as to produce a second coherent pulse stream; and cross-correlating the first coherent pulse stream with the second coherent pulse stream to produce an output which is proportional to the temporal displacement between the optical pulses in the two streams.